In a number of situations exchange of power must be performed between AC networks with different or at least not synchronous frequencies. The most frequent cases are the following:                1. Connection of not synchronous three phase networks with equal rating frequencies, e.g. between eastern and western Europe.        2. Connection of three phase networks with different frequencies, most usually 50 Hz/60 Hz (e.g. Japan, Latin America).        3. Connection of a three phase network and a low frequency, one/two phase network for railway supply, in Europe 50 Hz/16.2/3 Hz, in USA 60 Hz/25 Hz.        4. The use of rotating asynchronous converters as a series compensation in long distance AC transmission.        
Today, the connection is performed with the aid of power electronics and DC intermediate link. In the above mentioned cases 2 and 3 the connection can further be performed with the aid of matrix converters. In case of synchronous, but different frequencies in the above mentioned cases 2 and 3 the connection can further be performed with the aid of rotating converters comprising mechanically connected synchronous machines.
In the article, “Investigation and use of asynchronized machines in power systems”, Electric Technology USSR, No. 4, pp. 90-99, 1985, by N. I. Blotskii, there is disclosed an asynchronized machine used for interconnection of power systems, or their parts, which have different rated frequencies, or the same rated frequencies, but differing in the degree of accuracy with which it must be maintained. The structure of the asynchronized machine is disclosed in FIG. 1. The asynchronized machine includes an electric machine 1 which is a machine with a conventional three-phase stator and either a non-salient-pole symmetrical rotor or a salient-pole or non-salient-pole electrically asymmetrical rotor, the phase leads being connected to slip rings; an exciter 2 which is a cycloconverter or reversing controlled rectifier, the cycloconverter supply 3 or 4, a regulator 5 forming the control law required for the rotor ring voltages and the main machine rotor angle and speed 6, voltage 7 and current 9 sensors of the stator and rotor.
In the article, “Performance Characteristics of a Wide Range Induction type Frequency Converter”, IEEMA Journal, Vol. 125, No. 9, pp. 21-34, Sep. 1995, by G. A. Ghoneem, there is disclosed an induction-type frequency converter as a variable frequency source for speed control drives of induction motors. In FIG. 2 there is disclosed a schematic diagram of the induction-type frequency converter. The induction-type frequency converter consists of two mechanically and electrically coupled wound rotor induction machines A, B. The stator windings of one of them (A) are connected to 3-phase supply at line frequency (Vi, Fi), while the stator windings of the other machine (B) represent the variable frequency output (Vo, Fo). The rotor windings 10, 12 of the two machines are connected together with special arrangement. The converter is driven by a variable speed primemover 14, a DC motor can be used.
Static converters have drawbacks such as relatively low efficiency (ca 95%) owing to the losses in the semiconductors, harmonics which have to be compensated with the aid of filters. The use of DC intermediate links leads to the use of special converter transformers with very complex design. The fillers are leading to a great need of space for the total assembly. Conventional rotating converters are not designed for high voltages, so a transformer is needed at each side for the connection to the AC network. The efficiency then becomes comparable to or even lower than the efficiency of a static converter.